(1) Field of the Invention
This invention relates to linear image sensors used to read pictures in such devices as facsimiles, image scanners, digital copiers, etc. More particularly, the invention concerns photo response uniformity among individual photo sensing elements.
(2) Description of the Related Art
Conventional linear image sensors, as shown in FIGS. 2(a)-(c), usually consist of a plurality of diffusion layers to store photo excited carriers by equal-area conversion. FIG. 2(a) shows the linear image sensors. In an n.sup.- type semiconductor substrate 12, p type high impurity concentration diffusion layers 14 are formed to form photo-diode light receiving elements. The region 11 is an n.sup.+ type semiconductor substrate. Inter-insulating film 13 consists of SiO.sub.2 for deactivating the surface of the substrate. Photo sensing elements 21 are all of the same size. Photo sensing elements 22 are end positioned elements and are of a different size than the other elements 21; however, the areas of the end positioned elements 22 as well as the other elements 21 are equal. As shown in FIG. 2(b), a.times.b=a'.times.b'. A portion 32 is between an element isolation layer 16 and a diffusion layer 31. A portion 33 indicates an electrode lead.
Such conventional linear image sensors discussed above are disclosed in Publication No. 57-157680.
In conventional linear image sensors, between the impurity diffusion layers 21 (which become normal photo sensing elements which store photo excited carriers) and the impurity diffusion layers 22 (which store photo excited carriers by equal-area conversion), regionally generated photo sensing carriers are the same in capacity, but different in shape. This means that the distance the carriers stored in the layer 22 must travel becomes greater than that for those stored in the layer 21. Thus, the field intensity of the layers 21 and 22 in the direction of the electrode lead 33 becomes different, and therefore, it takes longer to remove or to read the carriers out from the layer 22. This difference in reading time in conventional linear image sensors has caused a problem. Photo sensing elements consisting of photo diodes somewhat help to reduce the difference in reading time; however, in photo sensing elements consisting of photo-transistors having differences in shape as shown in FIG. 2(b), i.e., wherein the width of b' is shorter than that of b, even though the areas of the two layers are equal, a problem exists wherein the amplification factor Hfe of the layer 22 becomes higher than that of the layer 21. As a result, the amount of signal electric charge of the layer 22 becomes larger than that of the layer 21, thereby resulting in problems of reducing photo response uniformity among the linear image sensor elements.
For uniform resolution and improved quality of images, all photo sensing elements would preferably be made equal in size and would be arranged keeping the distance between each of the elements equal.
According to the present invention, and in order to solve the above-mentioned problems, objectives of the invention are to improve photo response uniformity in linear image sensors by means of making all photo sensing areas equal in area by equal-area conversion. In addition, the shape of all impurity diffusion layers used to store generated photo excited carriers are made equal.
The present invention has an untransmittable (or non-light transmittable) shading film having photo-sensing windows or opening holes to allow photo sensing elements to receive light, while other areas are shaded from receiving light. The area of each of the opening holes is made to be equal. All diffusion layers for storing generated photo excited carriers are also made to be equal in both shape and area. Thereby, the present invention improves photo response uniformity.
A plurality of linear image sensors is formed at the same time in the surface of a semiconductor substrate using a conventional fabrication technique to produce solid state imaging devices, as shown in FIG. 7. With regard to individual image sensors prior to isolation of each one of the plurality of sensors formed at the same time, since there are no grooves at the boundaries of each individual linear image sensor 50, the photo-electro conversion characteristics are inspected by irradiation to the surface of the semiconductor wafer.
In the conventional method of inspecting linear image sensors, as shown in FIG. 8, which is a sectional view taken in the direction of the arrows substantially along the line B--B' of FIG. 7, each portion representing boundaries 44 between each of a plurality of linear image sensors 50 is irradiated by light. Photo sensing carriers 62 generated in the boundary region of a semiconductor wafer are entered into a diffusion layer 43 which forms the photo sensing element of the linear image sensor being inspected. This results in an increase of photo electro conversion efficiency. In the worse case, since linear image sensors adjacent to the one being inspected are also irradiated by light, more photo sensing carriers 61 are generated and enter into the diffusion layer 43 which forms the photo sensing element of the linear image sensor 50 being inspected. The result is akin to an increase of photo-electro conversion efficiency. However, the inspected linear image sensors are used in the form of multi-chip and contact type linear image sensors consisting of a plurality of linear image sensors, and in the final stage each individual linear image sensor is completely cut off and is used separately. Thereby, under the condition that each individual is completely separated, photo-electro conversion characteristics of photo sensing carriers 45 generated in each linear image sensor by irradiation of light through a photo sensing window 42 reflects photo-electro conversion efficiency of contact type linear image sensors. Therefore, in the conventional method of inspection, conversion efficiency results were largely different before and after cutting off each individual image sensor from the wafer. In the worst case, the conversion efficiency of individually formed and separated contact type linear image sensors was shown to be about 20 to 60 percent lower compared to that of the image sensors while still on the semiconductor wafer.
According to the present invention, in order to solve the above-mentioned problems, an objective is to obtain consistent photo-electro conversion efficiency at the time of inspecting linear image sensors formed on the semiconductor wafer.